1. Field of the Invention
This invention relates to uninterrupted power supply (UPS) apparatus and, more particularly, to a power factor corrected UPS maintaining integrity of the connection from power line neutral to an output load terminal.
2. Description of the Prior Art
UPS systems are now widely used to provide a secure supply of power to critical loads such as computers, so that if the line voltage varies or is interrupted, power to the load is maintained at an adequate level and is not lost. The UPS conventionally comprises a rectifier circuit for providing a DC voltage from the AC power lines; an inverter for inverting the DC voltage back to an AC voltage corresponding to the input, for delivery to the load; and a battery and a connection circuit for connecting battery power to the input of the DC to AC inverter, so that when reliable AC power is lost the delivery of AC power to the load is substantially unaffected. In such an UPS, it is highly desirable to maintain an uninterrupted neutral from the commercial AC utility power to each component circuit and to the load, e.g., in order to eliminate shock hazards. Because of the inherent nature and mode of operation of typical UPS systems, conventional UPS designs did not maintain the integrity of the neutral through the processing circuitry, requiring some type of isolation means such as isolation transformer to re-establish the neutral at the load. U.S. Pat. No. 4,935,861, assigned to the assignee of this invention, provides an UPS wherein the electrical continuity of an electrical conductor is maintained from one terminal of the AC utility through to one of the load terminals, without any isolation means being required.
The problem with maintaining integrity of the neutral is further complicated in a UPS having a power factor correction circuit. The task of connecting the battery to neutral is simple in a power supply unit without a PFC circuit, such as shown in U.S. Pat. No. 4,823,247. But as is well known, there are important reasons for incorporating power factor correction (PFC) into an UPS. And, the incorporation of such a PFC circuit imposes additional difficulties upon the goal of maintaining integrity of a neutral connection from the power line to the load. A design for achieving an uninterrupted power supply system having a PFC circuit is disclosed in U.S. Pat. No. 4,980,812, also assigned to the assignee of this invention.
It is recognized that maintaining the integrity of the neutral in an UPS offers advantages of lower cost, due to lack of need for isolation means, and higher reliability. Because of the design criterion of an undisturbed neutral, an UPS with a PFC circuit has heretofore required three converters. As seen in FIG. 1, such a prior art apparatus contains a converter as part of the power factor correction circuit, the output of which provides DC on a positive high voltage (HV) rail and independent negative HV rail respectively relative to the neutral line. The DC-AC inverter is necessarily a second converter, and, a third converter circuit has been necessary to connect the DC from the battery to the HV rails. Prior art attempts to combine the battery converter with the PFC converter have always resulted in either an isolated UPS, wherein the neutral is not maintained, or some circuit arrangement for connecting the DC output of the battery into an AC voltage which could be utilized by the AC to DC converter portion of the PFC circuit. For safety reasons, it is desirable to effectively connect the battery to the neutral, which leaves an unfulfilled need for an efficient and reliable manner of translating the battery output to the HV rails. The design solution of having a third converter of some different kind, or the option of using an isolation transformer, both have obvious disadvantages. The problem is thus how to provide that the converted output from the PFC circuit, as well as the battery output, can be independently loaded and still balanced around neutral to the plus and minus HV rails without using a separate converter of some sort for each. Stated differently, the problem for which a solution has not heretofore been known is how to connect the battery to the HV rails utilizing the PFC converter, while effectively maintaining a connection from the battery to neutral.